As interest related to Wireless Broadband (WiBro) systems, which use a 2.3 GHz band for Internet Protocol (IP)-based packet services, and 4th-generation (4G) mobile communication systems increases, active research is being conducted in order to satisfy requirements of the WiBro systems and the 4G mobile communication systems in relation to cell coverage and initial investment costs. Cell coverage may generally be affected by the efficiency of a control channel for synchronizing.
In particular, an orthogonal frequency division multiplexing (OFDM) technique has been proposed as a next-generation broadband wireless multiple access technique because the OFDM technique has strong characteristics against frequency-selective fading and narrow-band interference. In the OFDM technique, signals are transmitted simultaneously using different subcarriers by multiple users. The OFDM technique can transmit signals efficiently, and can compensate for channel distortions using a simple single-tab equalizer. In addition, the OFDM technique can solve intersymbol interference (ISI) problems easily using a cyclic prefix (CP). These advantages of the OFDM technique are why the OFDM technique is a suitable technique for 4G mobile communication systems.
Based on these advantages of the OFDM technique, active research is being conducted on an orthogonal frequency division multiple access (OFDMA, including OFDM-frequency division multiple access (OFDM-FDMA), OFDM-code division multiple access (OFDM-CDMA), and OFDM-time division multiple access (OFDM-TDMA)) system. The OFDMA system is a multi-user access system capable of satisfying various levels of quality of service (QoS). The OFDMA technique (or system) allocates different subcarriers to multiple users, respectively. Thus, the OFDMA technique can ensure various levels of QoS by allocating various resources according to requirements of users. The OFDMA technique has also been proposed as a next-generation broadband wireless multiple access technique because the OFDMA technique has strong characteristics against frequency-selective fading and narrow-band interference. The OFDMA technique uses a physical layer of the Institute of Electrical and Electronics Engineers (IEEE) 802.16a standard. The OFDMA technique is becoming highly associated with high-speed mobile Internet techniques being researched in South Korea. In addition, the OFDMA technique can be employed in 802.16e fixed or mobile Worldwide Interoperability for Microwave Access (WiMAX) systems or in 802.11 systems.
In mobile communication systems, a signal-to-noise ratio (SNR) of a received signal is an important estimated value, and the estimated value is used for various purposes. For example, the SNR can be a measure of quality for handoff signals. A receiver can use the SNR in adaptive modulation and coding (AMC) after receiving the SNR (as feedback) related to current wireless conditions. The SNR can be used for calculating a log-likelihood ratio for the receiver. In addition, the SNR can be used for a power control technique, a call setup technique, a diversity technique, etc.
In order to estimate the SNR, the transmitter has to transmit signals using predetermined data patterns that the receiver can recognize. The signals may include a preamble or a pilot so that the receiver may estimate the SNR using the preamble or the pilot.
For example, a WiBro system using the OFDM modulation technique uses three segments. A preamble data sequence having a length of 284 according to each segment is transmitted using the first OFDM symbol of a down-link frame.
When N of the predetermined data that the receiver can recognize are transmitted from the transmitter and pass through a fading channel, received signals that the receiver receives are expressed as [Expression 1].yk=Hkxprmb,k+nk k=0, 1, . . . , N−1  [Expression 1]
(Here, k denotes the subcarrier index of the OFDM, yk denotes received signals in a kth subcarrier in a frequency domain of the OFDM, Hk denotes a channel used by the kth subcarrier, xprmb,k denotes a signal in the frequency domain transmitted from a transmitter, and nk denotes a noise signal added to the kth subcarrier.)
If |xprmb,k|2=1, an estimated SNR is expressed as [Expression 2].
                              S          ⁢                                          ⁢          N          ⁢                                          ⁢          R                =                              E            ⁡                          (                                                                                      H                    k                                                                    2                            )                                            E            ⁡                          (                                                                                      n                    k                                                                    2                            )                                                          [                  Expression          ⁢                                          ⁢          2                ]            
In [Expression 1], xprmb,k is a predetermined data pattern. Hk and nk are unknown values. Thus, the SNR is estimated under conditions in which two unknown variables exist per each subcarrier.
Usually, methods for estimating the SNR are divided into a method using a channel estimation result and a blind-type algorithm that does not use a channel estimation result.
In the method using a channel estimation result, noise power is estimated by calculating Ĥk first and by calculating nk later using [Expression 3].
                                          n            k                    =                                                    y                k                            -                                                                    H                    ^                                    k                                ⁢                                  x                                      prmb                    ,                    k                                                  ⁢                                                                  ⁢                k                                      =            0                          ,        1        ,        …        ⁢                                  ,                  N          -          1                                    [                  Expression          ⁢                                          ⁢          3                ]                                          E          ⁡                      (                                                                            n                  k                                                            2                        )                          =                                            ∑              k                        ⁢                                                                            n                  k                                                            2                                =                                    ∑              k                        ⁢                                                                                                y                    k                                    -                                                                                    H                        ^                                            k                                        ⁢                                          x                                              prmb                        ,                        k                                                                                                                        2                                                          [                  Expression          ⁢                                          ⁢          4                ]            
The efficiency of the method using the channel estimation is determined by the accuracy of Ĥk. That is, the efficiency of the SNR estimation is determined by whether Ĥk is estimated accurately.
The noise power cannot be estimated by a usual least square method having low complexity. If Ĥk is estimated using the least square method, Ĥk is estimated using [Expression 5]. Thus, the result is zero when [Expression 5] is substituted with [Expression 4], and thus the noise power cannot be estimated.
                                          H            ^                    k                =                                                            (                                  x                                      prmb                    ,                    k                                                  )                            *                                                                                        x                                      prmb                    ,                    k                                                                              2                                ⁢                      y            k                                              [                  Expression          ⁢                                          ⁢          5                ]            
Therefore, the channel has to be estimated by reducing much noise using a minimum mean square error (MMSE) method or a method having high complexity. In addition, the noise power has to be estimated using the estimated channel.
The blind-type algorithm is a method that estimates the noise power without estimating Ĥk. In a wireless local area network (WLAN) system in which the preambles are in two OFDM symbols, the noise power can be estimated using [Expression 6] if the channel is riot changed on a time axis.
                              E          ⁡                      (                                                                            n                  k                                                            2                        )                          =                              1                          2              ⁢              N                                ⁢                                    ∑              k                        ⁢                                                                                                y                                          0                      ,                      k                                                        -                                      y                                          1                      ,                      k                                                                                                  2                                                          [                  Expression          ⁢                                          ⁢          6                ]            
(Here, y0,k denotes received signals in a kth subcarrier of the first OFDM symbol, and y1,k denotes received signals in a kth subcarrier of the second OFDM symbol.)
The estimation of the noise power using [Expression 6] is based on the assumption that y0,k and y1,k use the same channel. Thus, the efficiency is low when the channel is changed on the time axis due to movements of the receiver, etc.
In an 802.16e mobile WiMAX system, only one OFDM symbol includes preamble data. Thus, the blind-type algorithm cannot be employed in the 802.16e mobile WiMAX system.
Therefore, the noise power may be estimated using [Expression 7] if the channel change is small on a frequency axis.
                              E          ⁡                      (                                                                            n                  k                                                            2                        )                          =                              1                          2              ⁢                              (                                  N                  -                  1                                )                                              ⁢                                    ∑              k                        ⁢                                                                                                y                    k                                    -                                      y                                          k                      +                      1                                                                                                  2                                                          [                  Expression          ⁢                                          ⁢          7                ]            
The efficiency of a method using [Expression 7] is low because the channel changes quickly on the frequency axis when the delay spread of the channel is long.